Carbon dioxide is of great interest as a solvent in chemical processing because it is non-flammable, relatively non-toxic, and naturally abundant. These “green” properties have prompted the development of a host of new applications for CO2, some of which were made possible by the discovery of functional groups that enable miscibility of various moieties with CO2 at moderate pressures. Development of CO2 surfactants, for example, allows for processes such as emulsion polymerization or dry cleaning with CO2.
CO2 has been extensively employed to recover oil from underground formations, as it is inexpensive, non-flammable, relatively non-toxic and remediation is not required. In enhanced oil recovery (EOR), a flooding agent is pumped into the oil-bearing formation to move the petroleum to exit wells (see for example U.S. Pat. Nos. 4,480,696, 4,921,635 and 5,566,470). Water is most often used as the flooding agent, yet intimate contact between petroleum and water creates cross-contamination that mandates remediation of large volumes of organic-contaminated water. Indeed, a life cycle analysis of polystyrene performed during the 1980's suggested that the extraction of petroleum from the ground produces more liquid waste than any other process step over the entire cradle-to-grave lifespan of the material. Carbon dioxide would be a more sustainable flooding agent than water, but the viscosity of CO2 is too low to efficiently recover petroleum from the formation. Rather than sweep the oil before it, carbon dioxide “fingers” its way through the petroleum and hence leaves most of the oil behind.
Researchers in the petroleum engineering field have tried for decades to design additives that can raise the viscosity of carbon dioxide (at low concentration) to levels that would render CO2-flooding more practical, but success has been elusive. Additives have been synthesized that enhanced the viscosity of simple hydrocarbons, yet which were not soluble in CO2 without the use of impractically high fractions of co-solvent. Other additives have been identified that were CO2-soluble but which did produce any changes in the viscosity of CO2.
Polymer-based surfactants have also been developed for use in increasing the viscosity of CO2 and/or CO2 solubility (WO 00/35998 and U.S. Pat. No. 6,686,438). However, these materials have not found much practical utility due to issues with their relative solubility in CO2 and water, specifically in their tendency to be hydrophilic but not very CO2-philic.
Improvement in the efficiency of CO2-flooding will promote the use of CO2 over water in EOR and thus reduce the volume of liquid waste produced during petroleum extraction. Use of CO2 in EOR also results in its sequestration in rock formations, potentially an important part of an overall CO2 sequestration strategy. Thus, what is at first glance simply a technical problem in petroleum engineering has significant environmental ramifications as well. This discussion highlights the need for compositions that increase the viscosity of fluids comprising supercritical CO2 and water.